A central assumption of deliberative theory is that political preferences are endogenous to decision-making processes in which they are transformed by communicative interaction. We identify discursiveness and coordination of interaction as central determinants of preference change and develop a typology of political modes of interaction that affect the likelihood of preference change differently. These properties are in turn influenced by institutional characteristics of the fora in which communicative interaction takes place. To illustrate our approach empirically we present a comparative analysis of two extreme modes of interaction, ‘debate’ and ‘deliberation’, providing a case study of a parliamentary debate and a citizen conference on the same conflict: the import of embryonic stem cells in Germany. We assess the discursiveness and coordination as well as the amount of preference transformation in both forums.